This invention relates in general to the construction of cathodes and in particular to a new and useful hairpin cathode which provides an electron source particularly for electron microscopes and similar electron-optical instruments.
Hairpin cathodes produced of wires of high-melting metals, in particular tungsten, are employed today in general as standard electron sources, for example, in electron microscopes and other electron-optical instruments.
Hairpin cathodes are used particularly in electron microscopes, in which a high beam value is required and, therefore, relatively high operating temperatures of 2700.degree. to 2800.degree. K are customary. Since the invention of the electron microscope the short operating life of the cathodes, which, as a rule, is only 20 to 50 hours, is a disturbing factor with which operators had to cope.
Intensive study of the factors, which influence the operating life of the cathode has shown, however, that possibilities exist of increasing the operating life, which have not been recognized and utilized so far, and the operating life can be increased several times without impairing the electronoptical properties.
The tungsten hairpin cathodes, which are applied today in electron microscopes, are manufactured of pure or thoriated tungsten wire of 0.12 to 0.14 mm diameter. The inner bending radius at the crown is most often 0.05 to 0.1 mm. This hairpin is connected at both of its leg ends by spot welding to the heating current feed lines in the cathode base. Both legs should be of such length, that the emitting crown point is at the same distance from the cold ends of the hairpin and, therefore, reaches the highest temperatures during operation. Therefore, the greatest removal of material through vaporization should also occur at this site and the operating life should be determined by the temperature and wire thickness at this site.
However, experience has shown that the cathode always melts next to the bend (see also FIG. 4). Hence a region next to the bend, apparently assumes higher temperatures than the emission center.
This can be observed, if a hairpin cathode is heated outside of the electron microscope in vacuo and the temperature distribution is observed with pyrometers over a period of time at constant crown temperature. Initially, no measurable temperature difference between the crown point and the immediately adjacent leg regions can be detected. Only after several hours can it be observed that one leg is noticeably hotter. The temperature difference subsequently becomes increasingly more pronounced until the wire melts thoroughly at the hottest site.
If it is recalled that at 2800.degree. K a temperature increase by 10 K causes an increase of the vaporization rate by approximately 12%, it becomes clear, that even slight asymmetries in the temperature distribution can have catastrophic effects.
Examining the energy balance, which is given for each part of the wire by the Joule's heat supplied as the heat lost through radiation and heat conduction along the wire, offers an explanation. It is found that in the hairpin cathodes customary until now it is, for reasons of energy availability, not even possible that the expected temperature maximum occurs at the crown point. If one considers a short wire section in the region of the bend, the added radiation of the adjacent wire section toward the inside of the bend is less and its radiation toward the outside is greater than in the adjacent leg regions. Consequently, a temperature distribution obtains such as is shown in FIG. 1 by the solid line. Here, the temperature T over the distance d left and right from the crown point is plotted. Due to the good heat conductivity of tungsten, the temperature sink at the crown point is pyrometrically barely measurable. It amounts to only a few degrees. Precondition for the same level of temperature maxima to the left and right of the crown point is that the heat balance in the two legs is exactly symmetrical. If this is not the case, then an asymmetric temperature distribution originates as shown by the dotted line. This asymmetry becomes increasingly more pronounced over the course of time, and the resistance increase, through vaporization on the one side, becomes increasingly greater and also the removal on the other side through lowered temperatures decreases if the temperature at the crown point is kept constant. The temperature difference will, as the dot-dash line in FIG. 1 shows, increase to the point of, catastrophic thorough melting of a leg.
The asymmetry of the temperature distribution can have several causes:
1. uneven length of the two legs;
2. poor welding, i.e. poor heat transition of a leg to the current feed line;
3. poor electrical contact and heat transition at one of the contact pins of the cathode base;
4. inhomogeneities in the cathode material;
5. Thomson effect, which, as a consequence of the temperatures gradient in the two legs, and, depending on the current direction, leads to a heat supply in one leg and heat removal in the other.
This effect under normal operating conditions is not negligible, since the temperature difference caused by it can be 20 to 30.degree. K.
These different causes can be additive with respect to their effect but can also completely or partially compensate each other. While causes 1 to 3 can be eliminated by careful manufacture of the cathodes and inhomogeneities in the cathode material are rare, the Thomson effect can only be eliminated through alternating current heating or through frequent polarity reversal of the current direction.,